In recently years, plastic insulating materials are expected as film materials for film condensers for communication, electronic devices, electric power, medium and low voltage phase advancement and inverter, piezoelectric devices, pyroelectric devices and dielectric materials for transfer printing carrier since they have a high insulation resistance, excellent frequency characteristics and excellent flexibility.
A film condenser is usually comprised of a film structure comprising a dielectric polymer film subjected to aluminum or zinc deposition on its surface, or a film structure comprising multi-layers of aluminum foils and dielectric polymer films, and recently there are used a lot of condenser films comprising a dielectric polymer film and an electrode formed thereon by metal deposition.
Hydrocarbon polymers such as polypropylene, polyester and polyphenylene sulfide are studied as a dielectric polymer for a film of film condenser. However a dielectric constant of such a film itself is only about 2.3 to about 3.
It is known that a capacity of a film condenser is proportional to a dielectric constant of a film and reversely proportional to a film thickness.
Therefore, making a film thinner has been studied, but if a film is made too thin, film formation becomes difficult, and lowering of a withstand voltage is recognized. Accordingly there is a limit in making a film thinner.
For making a dielectric constant of a film itself higher, highly dielectric polyvinylidene fluoride polymer and cyano-ethylated pullulan have been studied as a polymer. However dielectric constants of any of these films are not more than 20, and those polymers are materials making it difficult to make a film thinner.
In recent years, making a size of a film condenser smaller and its capacity larger has been advanced, and as a result, it is strongly demanded to make a dielectric constant higher.
As one of means for making a dielectric constant of a film condenser higher, it is proposed to incorporate inorganic particles having an especially high dielectric constant with a polymer to make a film.
For mixing highly dielectric inorganic particles to a polymer and making a film, there are known (1) a melt-kneading method and (2) a coating method.
The melt-kneading method (1) is a method of kneading highly dielectric inorganic particles and a polymer at a temperature of not less than a melting temperature of the polymer, making the mixture into a film by a melt-extrusion method or an inflation method, and if necessary, subjecting the film to stretching treatment. In this method, there are known use of a hydrocarbon polymer such as polyphenylene sulfide, polypropylene or polyester (JP2000-501549A and JP2000-294447A) and use of a vinylidene fluoride polymer (for example, JP59-43039A, JP60-185303A and JP58-69252A). However in these methods, it is difficult to produce a thin film having a high dielectric constant and few voids.
The coating method (2) is a method of producing a film by dissolving a polymer in a solvent, adding and mixing thereto highly dielectric inorganic particles to make a coating composition, and then forming a film by a coating method.
In the coating method (2), there are known use of rigid hydrocarbon polymers being excellent in heat resistance and mechanical strength such as aromatic polyamide, aromatic polyimide and an epoxy resin (JP2001-106977A, JP1-248404A, JP4-160705A, JP2-206623A and JP2002-356619A) and use of a vinylidene fluoride polymer (JP54-129397A).
In the case of using a rigid hydrocarbon polymer, it is possible to produce a thin film having a high mechanical strength but there is a large effect of a dielectric constant of the polymer itself, thereby limiting improvement of a film dielectric constant, and a film becomes hard. Therefore, it cannot be said that such a film is suitable as a film for a film condenser which is required to have excellent winding property (flexibility). A system obtained by adding and mixing highly dielectric inorganic particles to a thermosetting resin is very hard, and is used for embedded capacitor, making use of its property of being hard.
In the case of using a vinylidene fluoride polymer, it is difficult to obtain a film in which highly dielectric inorganic particles are homogeneously incorporated in a polymer, and further improvement in making a thin film and increasing a dielectric constant is required.
Further it is proposed to produce a sheet comprising highly incorporated highly dielectric inorganic particles by press-molding composite particles prepared by coating highly dielectric inorganic particles with a vinylidene fluoride polymer (JP61-224205A). However the film is as thick as 150 μm, and cannot be said to be suitable as a film for a film condenser which is required to have excellent winding property (flexibility).